The expression "butyl rubber" as employed herein is defined to include a copolymer of a C.sub.4 to C.sub.6 isolefin, preferably isobutylene, and a C.sub.4 to C.sub.8 conjugated diolefin, preferably isoprene. A preferred butyl rubber contains from about 97 to about 99.5 weight percent of isobutylene and from about 0.5 to about 3 weight percent of isoprene.
The processes used to prepare butyl rubber are well known in the art. Commercially butyl rubber is prepared in a low temperature cationic polymerization process using Lewis acid type catalysts of which a typical example is aluminum trichloride. The process used most extensively employs methyl chloride as the diluent for the reaction mixture and the polymerization is conducted at temperatures of the order of less than -90.degree. C., the polymer produced thereby being obtained as a slurry. However, it is also possible to conduct such polymerizations in a diluent which is a solvent for the polymer produced, examples of such diluents being the hydrocarbons pentane, hexane, heptane and mixtures of the aforementioned solvents with one another or with methyl chloride and/or methylene chloride. Recovery of the butyl rubber is achieved by conventional techniques used to recover rubbery polymers.
Halogenated butyl rubber which can be either brominated butyl rubber or chlorinated butyl rubber is produced by the bromination or chlorination of the aforementioned butyl rubber. Brominated butyl rubber (bromobutyl rubber) typically contains from about 1 to about 3 weight percent of isoprene and from about 97 to about 99 weight percent of isobutylene, based on the hydrocarbon content of the rubber, and from about 1 to about 4 weight percent of bromine, based on the bromobutyl rubber. Chlorobutyl rubber typically contains from about 1 to about 3 weight percent of isoprene and from about 97 to about 99 weight percent of isobutylene, based on the hydrocarbon content of the rubber and from about 0.5 to about 2.5 weight percent of chlorine, based on the chlorobutyl rubber.
Commercially halogenation of the butyl rubber is carried out in a hydrocarbon solution such as hexane using elemental chlorine or bromine in a 1:1 molar ratio relative to the enchained isoprene in the butyl feedstock. In the process the solution of butyl rubber of the desired molecular weight and mole percent unsaturation is treated with chlorine or bromine at a temperature of from about 40 to about 65.degree. C. in one or more highly agitated reaction vessels, the chlorine being introduced as a gas or in dilute solution because of its rate of reaction with butyl rubber. Because of its lower rate of reaction bromine may be used in liquid or gaseous form. The hydrochloric or hydrobromic acid generated during the halogenation is neutralized with dilute aqueous base and the aqueous layer is subsequently removed by settling. Antioxidant or stabilizers are then added and the halogenated butyl rubber is then recovered in a manner similar to that used to recover butyl rubber.
Investigations of the molecular structure of the halogenated butyl rubbers have shown that in current commercial halogenation procedures a number of allylic halides are produced, the major halogenated isomer produced both in the case of chlorobutyl rubber and bromobutyl rubber being the exomethylene allylic halide isomer.
It is believed that these allylic halide structures in the halogenated butyl rubbers are the reason why the halogenated butyl rubbers exhibit enhanced cure compatibility with highly unsaturated elastomeric materials such as natural rubber, styrene-butadiene rubbers, polybutadiene rubbers and the like relative to butyl rubber.
In view of the foregoing it would be advantageous if halogenated polymers with allylic halide functionality analogous to the halogenated butyl rubbers could be synthesized directly by the copolymerization of the appropriate monomers thereby leading to a reduction in costs associated with the process of halogenating butyl rubber.